The xylene isomers are important intermediates which find wide and varied application in chemical syntheses. Para-xylene is a feedstock for terephthalic acid which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production.
The proportions of xylene isomers obtained from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene which is difficult to separate or to convert. Para-xylene in particular is a major chemical intermediate with rapidly growing demand, but amounts to only 20-25% of a typical C8-aromatics stream. Adjustment of isomer ratio to demand can be effected by combining xylene-isomer recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Isomerization converts a non-equilibrium mixture of the xylene isomers which is lean in the desired xylene isomer to a mixture approaching equilibrium concentrations.
One problem in such a process loop comprising para-xylene recovery and C8-aromatics isomerization is the presence of C9 and heavier aromatics (“C9+aromatics”) in the feed stream to the process combination. Generally the presence of substantial C9+aromatics in the isomerization or separation processes is unacceptable or undesirable from the standpoint of process performance or catalyst/adsorbent life. Usually the feed stream is fractionated to remove C9+aromatics before being sent to the process loop, an expensive step since the entire C8-aromatics portion must be lifted overhead as well as achieving the separation between the heaviest C8s and lightest C9+s. Avoidance of the resulting expensive energy and investment costs would be an advantage.
Various catalysts and processes have been developed to effect xylene isomerization, and these usually are differentiated by the manner of processing ethylbenzene associated with the xylene isomers. Ethylbenzene is not easily isomerized to xylenes, and separation from the xylenes by superfractionation or adsorption is very expensive. Isomerization of a mixture of xylenes and ethylbenzene followed by recovery of para-xylene and recycle of the resulting C8-aromatic raffinate thus tends to result in a buildup of the ethylbenzene concentration in the recycle. A widely used approach is to dealkylate ethylbenzene to form principally benzene while isomerizing xylenes to a near-equilibrium mixture. An alternative approach is to react the ethylbenzene to form a xylene mixture via conversion to and reconversion from naphthenes in the presence of a solid acid catalyst with a hydrogenation-dehydrogenation function. Some combination of these approaches can be found in the art.
Processes have been disclosed during the past quarter-century or so using one or more molecular-sieve catalysts for xylene isomerization. For example, U.S. Pat. No. 3,856,872 (Morrison) teaches xylene isomerization and ethylbenzene conversion with a catalyst comprising ZSM-5, -12, or -21 zeolite. U.S. Pat. No. 3,948,758 (Bonacci et al.) discloses the processing of an aromatics-rich reformate stream by hydrocracking, fractionation to separate benzene, toluene and C9+aromatics, separation of a desired isomer from the C8 aromatics and isomerization of the hydrocarbons lean in the desired isomer. U.S. Pat. No. 4,899,011 (Chu et al.) teaches isomerization of C8 aromatics using two zeolites, each of which is associated with a strong hydrogenation metal. U.S. Pat. No. 5,977,420 (Abichandani et al.) discloses a processing scheme in which a C8+feed is subjected to ethylbenzene conversion followed by fractionation to remove C9+, with the overhead processed in a loop comprising a benzene/toluene column, para-xylene recovery, and isomerization with the isomerate returned to the benzene/toluene column. U.S. Pat. No. 6,222,086 (Sharma et al.) teaches the use of two zeolitic catalysts for the isomerization of a mixture of xylenes and ethylbenzene wherein the content of platinum-group metal in the second catalyst is no more than about 30% of that in the first catalyst. U.S. Pat. No. 6,448,459 (Magne-Drisch et al.) discloses a process combination comprising recovery and isomerization of a first fraction of enriched ethylbenzene concentrate, recovery of para-xylene by adsorption from the second fraction from ethylbenzene enrichment, and isomerization of raffinate and desorbent from the para-xylene adsorption step. U.S. Pat. No. 6,660,896 (Buchanan et al.) teaches a process for isomerizing a feed containing ethylbenzene and a mixture of xylene isomers using first and second catalysts in the presence of hydrogen to produce a product having higher-than equilibrium para-xylene. Although these references teach individual elements of the present invention, none of the art suggests combination of the elements to obtain the critical features of the process of the present invention.
None of the art suggests the present efficient process and catalyst combination for obtaining paraxylene from a C8+feedstock having a substantial content of C9+aromatics.